Fast Wireless Handoff in 802.11 Networks

1
Fast Wireless Handoff in 802.11 Networks

• Sangho Shin
• Andrea G. Forte
• Anshuman S. Rawat
• Henning Schulzrinne

2
802.11 Networks

• A handoff occurs when a mobile station moves
  beyond the radio range of one AP and enters
  another BSS.

3
Handoff delay
Mobile station
All APs
Probe request(broadcast)
Probe response
Probe delay
New AP
Authentication request
Authentication delay
Authentication response
Reassocation request
Reassociation delay
Reassociation response
4
Handoff Delay
5
Problems

• Handoff latency is too big for VoIP.
• Seamless VoIP requires less than 90ms latency.
• Handoff delay is from 200ms to 400msec.
• The biggest component of handoff latency is
  probing (over 90).

Solutions

• Selective Scanning
• Caching

6
Selective Scanning

• In most of the environments (802.11b 802.11g),
  only channel 1, 6, 11 are used for APs.
• Two APs that have the same channel are not
  adjacent (Co-Channel interference).
• Scan 1, 6, 11 first !!

7
Selective Scanning - Algorithm
Channel mask 1, 6, 11
Scan channels using channel mask
Found good AP ?
Yes
Join to the AP
No
Flip channel mask
Set new channel mask
New channel mask 1,6,11 scanned channels
current channel
8
Caching

• Background
• Spatial locality
• Algorithm
• After scanning, store the candidate AP info into
  cache (keycurrent AP)
• Use the AP info in cache for association without
  scanning when handoff happens

Key AP1 AP2
1 Current AP Next best AP The second best AP
. . .
N
9
Cache Simulation
Key Best Next
1 AP1(11)
2
Coverage area
Detection area
1, 6
Channel mask
AP1(11)
Handoff !!
AP3(1)
AP2(6)
10
Cache Simulation
Key AP1 AP2
1 AP1(11)
2 - -
Coverage area
Detection area
1, 6
Channel mask
AP1(11)
Handoff !!
AP3(1)
AP2(6)
Cache miss!
Selective Scanning
AP2, AP3
11
Cache Simulation
Key AP1 AP2
1 AP1 (11) AP2 (6) AP3 (1)
2 AP2 (6) - -
Coverage area
Detection area
1, 6
1, 11
Channel mask
AP1(11)
Handoff !!
AP3(1)
AP2(6)
Cache miss!
Selective Scanning
AP2, AP3
Connect
AP2
12
Cache Simulation
Key AP1 AP2
1 AP1 (11) AP2 (6) AP3 (1)
2 AP2 (6) -
Coverage area
Detection area
1, 11
Channel mask
Handoff !!
AP1(11)
AP3(1)
AP2(6)
13
Cache Simulation
Key AP1 AP2
1 AP1 (11) AP2 (6) AP3 (1)
2 AP2 (6) -
Coverage area
Detection area
1, 11
Channel mask
Handoff !!
AP1(11)
Cache miss!
AP3(1)
AP2(6)
14
Cache Simulation
Key AP1 AP2
1 AP1 (11) AP2 (6) AP3 (1)
2 AP2 (6) AP1 (11) -
Coverage area
Detection area
1, 11
Channel mask
Handoff !!
AP1(11)
Cache miss!
AP3(1)
AP2(6)
Selective Scanning
AP1
15
Cache Simulation
Key AP1 AP2
1 AP1 (11) AP2 (6) AP3 (1)
2 AP2 (6) AP1 (11) -
Coverage area
Detection area
1, 11
1, 6
Channel mask
Handoff !!
AP1(11)
Cache miss!
AP3(1)
AP2(6)
Selective Scanning
AP1
Connect
AP1
16
Cache Simulation
Key AP1 AP2
1 AP1 (11) AP2 (6) AP3 (1)
2 AP2 (6) AP1 (11) -
Coverage area
Detection area
1, 6
Channel mask
AP1(11)
Handoff !!
AP3(1)
AP2(6)
17
Cache Simulation
Key AP1 AP2
1 AP1 (11) AP2 (6) AP3 (1)
2 AP2 (6) AP1 (11) -
Coverage area
Detection area
1, 6
Channel mask
AP1(11)
Handoff !!
AP3(1)
AP2(6)
Cache HIT!
18
Cache Simulation
Key AP1 AP2
1 AP1 (11) AP2 (6) AP3 (1)
2 AP2 (6) AP1 (11) -
Coverage area
Detection area
1, 11
Channel mask
AP1(11)
Handoff !!
AP3(1)
AP2(6)
Cache HIT!
Connect
AP2
19
Cache Simulation
Key AP1 AP2
1 AP1 (11) AP2 (6) AP3 (1)
2 AP2 (6) AP1 (11) -
Coverage area
Detection area
AP1(11)
1, 11
Channel mask
Handoff !!
AP1(11)
Cache Hit!!
AP3(1)
AP2(6)
20
Cache Simulation
Key AP1 AP2
1 AP1 (11) AP2 (6) AP3 (1)
2 AP2 (6) AP1 (11) -
Coverage area
Detection area
1, 11
Channel mask
Handoff !!
AP1(11)
Cache Hit!!
AP3(1)
AP2(6)
Connect
AP1
21
Expreriment Result Handoff time
22
Expreriment Result Packet Delay
23
Expreriment Result Packet Loss
24
Expreriment Result Packet Loss
25
Conclusion

• Fast MAC Layer handoff using selective scanning
  and caching
• Selective Scanning 100130 msec
• Caching 24 msec

• Dont need to modify AP, infrastructure, or
  standard. Just need to modify wireless card
  driver!